Along with an increase in the internet traffic, a larger capacity in an optical communication system of a trunk line system is demanded, and research and development are carried out on an optical transmitter receiver capable of transmitting a signal exceeding 100 [Gbit/s] per wavelength. However, in the optical communication, when a bit rate per wavelength is increased, a degradation in a signal quality becomes large because of an decrease in the bearing force for Optical Signal Noise Ratio (OSNR), the wavelength dispersion in a transmission path, the polarized wave mode dispersion, or the waveform distortion from an nonlinear effect or the like.
For this reason, in recent years, a digital coherent reception system having the OSNR bearing force and the waveform bearing force in the transmission path attracts attention (for example, see D. Ly-Gagnon et al, IEEE JLT, vol. 24, pp. 12-21, 2006). Also, in contrast to a system for the direct detection by assigning ON/OFF of a light intensity to binary signals in a related art, according to the digital coherent reception system, a light intensity and phase information are extracted through the coherent reception system. Then, the extracted intensity and phase information are quantized by an ADC (Analog/Digital Converter) and demodulated by a digital signal processing circuit (for example, see F. M. Gardner, “A BPSK/QPSK timing-error detector for sampled receivers”, IEEE Trans. Commun., vol. COM-34, pp. 423-429, May 1986).
However, according to the related art technology, when the frequency of the local light in the digital coherent receiver varies with respect to the frequency of the optical light transmitted from the transmitter, the optical signal cannot be digitally demodulated at a satisfactory precision in the digital coherent receiver. For this reason, a problem occurs that a communication quality is degraded.